Local weather phenomena Flashcards
What are Anabatic winds?
Anabatic winds arise when a mountain or hillside is exposed to direct sunlight. The surface warms faster than the valley or any opposing shaded slopes. The heated slope warms air in contact with it causing it to expand and rise. This, in turn draws in more cool air from the valley floor or in a circulation pattern like the one shown.
What are Katabatic winds?
Katabatic winds occur as a result of rapid radiation of heat from exposed mountain or hill slopes at night. This is particularly true of surfaces which are not covered with vegetation. Once the slopes have cooled they cool the air in contact with the slope causing it to increase in density and sink. This process draws in more air from above and displaces warmer air in the valley as shown below.
What are Sea breezes?
Sea breezes arise during the day because land masses warm up much more rapidly than adjoining
water masses. As the land warms, it warms the air in contact with it causing the air density to drop.
The lower density air is more buoyant than the surrounding air and thus rises. As the warmed air
rises, its place is taken by the cooler air from over the water setting up a local circulation pattern
What are land breezes?
At night land will cool off more rapidly than an adjoining body of water. This cools the air in contact with it, increasing its density and causing it to sink. The reverse process is set up as shown to the right. These are known as land breezes and are usually more gentle than sea breezes.
What are Chinooks?
Chinooks occur when air containing a significant amount of moisture blows towards a mountain range and is forced to rise. In the process, the air cools adiabatically, first at the dry rate of 3°C per 1,000 feet and if the mountain range is high enough, the moisture will condense reducing the rate of cooling to 1.5°C per 1,000 feet. In this situation, clouds will form and precipitation will occur on the windward side as shown, reducing the moisture content of the air. On the leeward side of the mountain the air will sink and warm adiabatically. Only now the air no longer contains the moisture so all the warming will be at the dry adiabatic lapse rate of 3°C per 1,000 feet. As a result, the air will warm substantially more when it descends than it did when it was forced to rise and cool.
What are Mountain waves?
When stable air is blowing towards a mountain range it is forced to rise. On the downwind or lee side of the mountains, the air descends and because of the terrain eddies and vortices are formed in the descent. In addition, the air compresses as it descends and then springs back. This creates an oscillating flow with its associated up and down drafts and often severe turbulence. If the air contains sufficient moisture, distinctive clouds will form in the regions where the pressure and temperature drop providing good warning of the existence of the phenomenon. Lenticular clouds will form at the crests of the waves of the oscillating flow while a rotor cloud will often form in the extreme turbulence of the eddies downwind of the mountain. Be warned though that the existence of these telltale signs is not a pre-requisite for the occurrence of mountain waves - they may be hidden by other clouds or if
the air is dry, they may not exist at all.
